This disclosure generally relates to tools for use in a borehole extending into a subterranean formation. More particularly, the disclosure relates to downhole tools for boosting annulus flow in the borehole as part of an oilfield drilling operation of a well system.
Drilling operations may produce a borehole having a cross-sectional diameter that varies along the borehole's length. Particularly, the borehole may have a diameter that is larger near the surface and is gradually reduced moving along the length of the borehole towards the toe or bottom of the borehole. For instance, the borehole diameter may change in size between casing or liner tubular members of different diameters that line the inner surface of the borehole. Some oilfield drilling operations include a drill string that extends through the borehole and terminates at a drill bit disposed at the bottom of the borehole for cutting into the subterranean formation into which the borehole extends.
In some such drilling operations, drilling fluid or mud may be pumped down through a central passage of the drill string from mud pumps disposed at the surface to the drill bit, where the pumped mud may cool the drill bit and circulate entrained drill cuttings to the surface through an annular flowpath formed between the borehole wall and the drillstring. Due to the varying cross-sectional diameter of the borehole along its axial length, the cross-sectional area of the annular flowpath may vary along the axial length of the borehole, with the annular flowpath having a larger cross-sectional area near the surface than towards the bottom of the borehole by the drill bit. As the drilling mud and entrained drill cutting flow upwards through the annular flowpath, the flow speed of the returning fluid, commonly known as annulus velocity (AV), may decrease in response to the increasing cross-sectional area of the annular flowpath moving towards the surface. Moreover, if the AV decreases by a sufficient degree, the AV may drop below the slip velocity of the returning fluid, causing the entrained drill cuttings to settle out of the recirculating mud, thereby inhibiting the recirculating mud from carrying the drill cuttings to the surface for removal from the borehole.